The present invention relates to a process for producing a hydrogen-containing gas by steam/hydrocarbon reforming and more particularly to a process where the overall steam-to-carbon molar ratio is below 2.5.
Hydrogen production by steam/hydrocarbon reforming, also called steam-methane reforming or SMR, is well-known. The process is an energy intensive process.
As energy costs rise, the hydrogen production industry has been driven to improve the thermal efficiency of the reforming process. Those skilled in the art recognize that the thermal efficiency improves when the overall steam-to-carbon molar ratio is reduced. Lower steam-to-carbon ratios reduce the waste heat loss from the reforming process and thereby enhance thermal efficiency. Industry has been successful reducing the steam-to-carbon molar ratio from about 3.0, which was conventionally used, to a value as low as 2.5.
One of the technical barriers for lowering the steam-to-carbon ratio is associated with downstream processing in a shift reactor, particularly where an iron-based high temperature shift catalyst is used. The shift reaction is used to convert the carbon monoxide in the reformer effluent with steam to produce more hydrogen. When the steam-to-carbon molar ratio is lower than about 2.8, the iron in the high temperature shift catalyst will be reduced to its lowest oxidation state. The reduced high temperature shift catalyst will catalyze undesired reactions such as converting hydrogen and carbon monoxide to hydrocarbons. Further, the reduced catalyst will lose its mechanical strength, causing the collapse of the catalyst bed and inoperable pressure drop through the catalyst bed.
This technical barrier has been overcome by using a copper-based medium temperature shift catalyst, which is insensitive to the overall steam-to-carbon molar ratio for the reforming process. Using a copper-based medium temperature shift catalyst, the steam-to-carbon molar ratio may be reduced below 2.5 without problems in the shift reactor.
As the steam-to-carbon molar ratio was lowered, industry experienced another technical barrier to decreasing the steam-to-carbon molar ratio. At steam-to-carbon molar ratios less than 2.5, carbon (coke or soot) would form on the reforming catalyst in the catalyst-containing reformer tubes in a top-fired reformer. Carbon formation deactivates and/or disintegrates the reforming catalyst, causing undesired pressure drop through the reformer tubes and/or overheating of the tubes. If the catalyst is deactivated and/or disintegrated, hydrogen production must be interrupted in order to regenerate or replace the catalyst.
Industry has handled this potential problem by recycling carbon dioxide and reducing the heat flux. However, carbon dioxide recycle is antithetical to hydrogen production. Reduced heat flux operation directly translates to increased number and/or length of expensive high alloy tubes in the reformer.
Carbon formation is affected by the temperature of the reforming catalyst in the fired reformer. The higher the temperature, the more severe the carbon formation. Since the catalyst is located inside the reformer tubes, the catalyst temperature is often expressed as the tube wall temperature. It is well known that top-fired reformers are more prone to carbon formation than side-fired reformers [T. S. Christensen, Applied Catalysis A: General 138 (1996) pages 285-309, Elsevier Science B.V.]. This is due to higher heat fluxes in the entrance section resulting in the higher tube wall temperatures thereby promoting carbon formation. As a result, no SMR processes using the top-fired reformer has used or been taught to use an overall steam-to-carbon ratio of less than 2.5.
Industry desires to improve the thermal efficiency of steam/hydrocarbon reforming processes. Industry desires a hydrogen production process using a steam/hydrocarbon reformer that can operate at high heat fluxes, but at efficiencies afforded by S/C operation below 2.5, and without the risk for carbon deposition.